Nickel-containing flanges for use in direct resistance heating of platinum-containing vessels

ABSTRACT

A flange ( 13 ) for use in direct resistance heating of a glass-carrying vessel ( 10 ), such as a finer, is provided. The flange comprises a plurality of electrically-conductive rings which include an innermost ring ( 140 ) which is joined to the vessel&#39;s exterior wall ( 12 ) during use of the flange and an outermost ring ( 150 ) which receives electrical current during use of the flange. The innermost ring ( 140 ) comprises a high-temperature metal which comprises at least 80% platinum and the outermost ring ( 150 ) comprises at least 99.0% nickel. This combination of materials both increases the reliability of the flange and reduces its cost. In certain embodiments, the flange can also include one or more rings ( 190 ) composed of a platinum-nickel alloy which has a lower thermal conductivity than platinum or nickel and thus can serve to reduce heat loss through the flange.

I. CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional claiming the benefit of priority under 35 U.S.C.§120 of U.S. patent application Ser. No. 12/217,656 filed on Jul. 8,2008, now U.S. Pat. No. 8,269,131 which claims the benefit of andpriority to U.S. Provisional Application Ser. No. 61/067,500 filed onFeb. 28, 2008, the contents of which are hereby relied upon andincorporated herein by reference in its entirety.

II. FIELD OF THE INVENTION

This invention relates to glass making and in particular to the directresistance heating of platinum-containing vessels used to hold ortransport molten glass, e.g., vessels such as melters, finers, stirchambers, formers, connecting tubes, and the like.

III. BACKGROUND OF THE INVENTION

As is well-known, platinum-containing materials, i.e., materials whichcontain at least 80 wt. % platinum, are widely used in the manufactureof glass and glass products because of their high melting temperatures,low levels of oxidation at elevated temperatures, resistance tocorrosion by glass melts, and low levels of contamination of moltenglass. As is also well-known, platinum-containing materials arenotoriously expensive. Accordingly, substantial reductions in capitalcosts can be achieved by even small reductions in the amount ofplatinum-containing materials used in a glass manufacturing facility.

Among the valuable characteristics of platinum-containing materials istheir ability to generate heat when conducting electricity. As a result,molten glass flowing through, or held in, a platinum-containing vesselcan be heated by passing electrical current between one or morelocations along the length of the vessel's exterior wall. Such heatingis known in the art as “direct heating,” “resistance heating,” or“direct resistance heating,” the term used herein.

A major challenge in direct resistance heating is the introduction andremoval of the electric current from the vessel's wall. This is not onlyan electrical problem, but is also a thermal problem since the materialsused to carry current to and from the wall will also conduct heat awayfrom the wall. As a result, cold spots can be generated at the wallwhich can be detrimental to the quality of the finished glass,especially for glasses having strict quality requirements, such as thoseused to make substrates for liquid crystal displays (LCDs).

One way of introducing current into a vessel's wall is through the useof an electrically-conductive flange. Examples of such flanges can befound in U.S. Pat. Nos. 6,076,375 and 7,013,677. The present inventionis concerned with flanges used to introduce current into aplatinum-containing vessel wall and, in particular, with the reliabilityand cost of such flanges.

IV. SUMMARY OF THE INVENTION

In accordance with one aspect, the invention provides a flange for usein direct resistance heating of a vessel (10) which, during use, carriesmolten glass and which comprises an exterior wall (12) which comprisesat least 80% platinum, said flange comprising:

a) a plurality of electrically-conductive rings e.g., in FIGS. 4-6,rings 140,141,142,150,151,190) which, during use, form a conductive pathfor carrying current to the exterior wall (12), said plurality of ringscomprising:

-   -   i) an innermost ring (140) which is joined to and electrically        connected with the vessel's exterior wall (12) during use of the        flange; and    -   ii) an outermost ring (150) which receives electrical current        during use of the flange; and

b) a cooling channel (160) associated with the outermost ring (150)through which a cooling fluid i.e., a liquid or a gas) flows during useof the flange;

wherein:

-   -   i) the innermost ring (140) comprises a high-temperature metal        which comprises at least 80% platinum; and    -   ii) the outermost ring (150) comprises at least 99.0 wt. %        nickel.

In certain embodiments, the cooling channel is a cooling tube thatcomprises at least 99.0 wt. % nickel. In other embodiments, theoutermost ring has a sufficient thickness so that during use of theflange, the variation in calculated radial electrical current densityalong the ring's inner periphery is less than fifty percent.

In accordance with another aspect, the plurality of electricallyconductive rings comprises a ring (190) which i) is located between theoutermost (150) and innermost (140) rings and ii) comprises aplatinum-nickel alloy, e.g., an alloy which comprises at least 77 wt. %platinum.

The reference numbers used in the above summaries of the various aspectsof the invention are only for the convenience of the reader and are notintended to and should not be interpreted as limiting the scope of theinvention. More generally, it is to be understood that both theforegoing general description and the following detailed description aremerely exemplary of the invention and are intended to provide anoverview or framework for understanding the nature and character of theinvention.

Additional features and advantages of the invention are set forth in thedetailed description which follows, and in part will be readily apparentto those skilled in the art from that description or recognized bypracticing the invention as described herein. The accompanying drawingsare included to provide a further understanding of the invention, andare incorporated in and constitute a part of this specification. It isto be understood that the various features of the invention disclosed inthis specification and in the drawings can be used in any and allcombinations.

V. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing illustrating the use of current-carryingflanges to heat the exterior wall of a glass-carrying vessel. The flangeincludes platinum-containing rings and a copper cooling tube.

FIG. 2 is a schematic drawing showing one of the flanges of FIG. 1.

FIG. 3 is a cross-section of the flange of FIG. 2.

FIG. 4 is a plan view of a flange which includes a platinum-containingring, a nickel-containing ring, and a nickel-containing cooling tube.

FIG. 5 is a cross-section of a flange which includes platinum-containingrings, nickel-containing rings, and a nickel-containing cooling tube.

FIG. 6 is a plan view of a flange which includes a platinum-containingring, a nickel-containing ring, a nickel-containing cooling tube, and aring which comprises a platinum-nickel alloy.

VI. DETAILED DESCRIPTION OF THE INVENTION AND ITS PREFERRED EMBODIMENTS

FIGS. 1-3 illustrate a direct resistance heating system employing acopper-based cooling/bus bar assembly. FIG. 1 shows a vessel 10 in thiscase a tubular vessel such as a finer) having an exterior wall 12 towhich are attached two flanges 13 which apply electrical current to wall12.

Although only two flanges are shown, in practice, multiple flanges canbe used for any particular vessel to provide electrical current todifferent sections of the vessel's exterior wall. Also, although theexterior wall in FIG. 1 has a circular shape, the wall can have avariety of other shapes, such as, elliptical, oval, square, rectangular,and the like. The central aperture of the flange will then have a shapesuitable for introducing current into the vessel's wall, preferablyaround its entire circumference. Although not shown in FIG. 1, duringuse, the vessel's wall and the flanges will normally be surrounded bythick layers of insulating refractory materials to control heat lossfrom the vessel.

FIGS. 2 and 3 show the construction of the flanges of FIG. 1 in moredetail. As can be seen in these figures, the flange includes two rings14,15 made of a platinum-containing material, e.g., platinum or aplatinum-rhodium alloy. These rings are welded to one another and theinner ring 14 is joined, e.g., welded, to exterior wall 12 of vessel 10.Rings 14 and 15 have different thicknesses, the inner ring 14 beingthicker than the outer ring 15. As discussed in more detail below, inthis way, the current density throughout the flange can be made lessthan the current density in the vessel's wall 12. Because the heatgenerated by electrical current is proportional to current density,keeping the current density in the flange less than the current densityin the vessel's wall minimizes heat generation in the flange.Accordingly, more direct electrical heating occurs in the vessel wallthan in the flange, as is desired.

In addition to rings 14 and 15, the flange of FIGS. 1-3, includescooling tube/circular bus bar 16 and main bus bar 17, both of which aremade of copper. The bus bar is electrically connected to a currentsource not shown) and the cooling tube is joined to ring 15 using silversolder. Water is circulated through the cooling tube to keep the tube,the main bus bar, and the silver solder at temperatures below those atwhich they will rapidly oxidize and/or melt. Substantial cooling isrequired because copper rapidly oxidizes at 400-500° C. and thetemperature of the molten glass in vessel 10 can be around 1600° C.

In addition to its cooling function, copper tube 16 also functions as abus bar to distribute current around the periphery of ring 15. Inparticular, tube 16 produces a current distribution that is sufficientlyuniform so as to minimize the formation of hot spots on the surface ofthe vessel near the location where the flange is joined to the vessel.In particular, hot spots develop in areas of high current density andcold spots in areas of low current density. Hot spots and cold spotsproduce temperature gradients which are undesirable in a glass deliverysystem since one of the sources of defects in finished glass, e.g.,finished glass sheets, is the presence of temperature gradients. Glassflow is also disrupted by temperature gradients.

In practice, it was found that the flange of FIGS. 1-3 can be improvedin a number of areas. For example, it can be difficult to produceconsistently good brazed/solder joints between copper tubing andplatinum. Also, cracking problems have been associated with direct weldsbetween copper and platinum. This cracking is believed to be due toundesirable phases that can form in a copper-platinum system see H. Luoand P. Duwez, “Solid Solutions of rhodium with copper and nickel”, J.Less common Metals, 1964, vol 6, pp 248-249).

More importantly, if water flow through tube 16 is interrupted for anyreason, an entire glass production line can be put at jeopardy in a veryshort period of time, e.g., 5-35 minutes. As the temperature rises, thecopper tube can oxidize to failure very quickly, which can generate acatastrophic water leak when the water flow is restored. Also, thesilver solder, which bonds the copper tube to ring 15, melts in a matterof minutes at elevated temperatures. In either case, the glassproduction line must be shut down so that the affected flange or flangescan be replaced or repaired. This represents a substantial loss inoutput, especially for high volume production lines such as those usedto produce LCD substrates where it can take days to weeks beforeproduction is returned to normal after a shut down.

In certain embodiments, the flanges of FIGS. 1-3 are improved throughthe employment in the flange of an outermost ring ring 150 in FIGS. 4-5)which comprises at least 99.0 wt. % nickel. In other embodiments, theflange also includes a cooling channel in the form of a cooling tube anda main bus bar, each of which comprises at least 99.0 wt. % nickel.

The nickel can, for example, be commercially pure nickel, such as,nickel 200 or nickel 201, which are readily available at low costcompared to platinum and platinum alloys. When used in a power flange,nickel provides an excellent combination of electrical resistance,thermal conductivity, oxidation resistance, solubility with platinum andrhodium, machinability, price, and availability in many forms andshapes, which other high temperature materials cannot match.

The use of nickel for the above components of a power flange has beenfound to significantly improve the ability of the flange to withstandtemporary stoppages in the flow of cooling water. In particular, theflange exhibits a high level of oxidation resistance so that if coolingwater flow is interrupted, the flange will remain operable for up toseveral days. The superior oxidation resistance of the nickel-containingflange provides sufficient time to restore coolant flow without loss ofthe platinum part and thus without the need to interrupt the flow ofglass through the vessel.

In addition to its ability to withstand temporary interruptions incoolant flow, nickel-containing flanges also require less cooling thancopper-containing flanges. Accordingly, in general, less directresistance heating is needed when a nickel-containing flange is used.This reduction in direct resistance heating, in turn, reducesoperational costs for electricity and capital costs in terms of thecapacity of the electrical source needed to power the direct heatingsystem.

In addition to these functional benefits, the use of one or more ringswhich comprise nickel significantly reduces the cost of the flange sincethe nickel is used at locations where platinum or a platinum alloy isused in a copper-containing flange. Although the prices of nickel andplatinum vary over time, as a rule of thumb, platinum is at least 400times more expensive than nickel and sometimes can be more than 1800times more expensive.

FIG. 4 shows a basic structure for a nickel-containing flange 130constructed in accordance with an embodiment of the invention. In thisfigure, reference number 150 is the outermost ring of the flange and isformed from commercially pure nickel, while reference number 140 is theflange's innermost ring and is formed of platinum or a platinum alloy,e.g., a platinum-rhodium alloy. More generally, ring 150 comprises atleast 99.0 wt. % nickel and ring 140 comprises a high-temperature metal(i.e., as used herein, a metal capable of operating at temperaturesabove 1500° C.) which comprises at least 80 wt. % platinum, with theremainder, if any, being one or more of: rhodium, iridium, gold, finelydivided metal oxides such as zirconium dioxide, and the like. As oneexample, ring 140 can comprise 90 wt. % platinum and 10 wt. % rhodium.

As also shown in FIG. 4, the flange can include a main bus bar 170,which is used to connect the flange to a power source (not shown), and acooling channel in the form of cooling tube 160. In an embodiment of theinvention, each of these components comprises at least 99.0 wt. % nickeland are welded to outermost ring 150. The cooling channel carries acooling fluid, which may be a liquid, e.g., water, or a gas, e.g., air.Although drawn as a separate component in FIG. 4, the cooling channelcan be formed in outermost ring 150 if desired, e.g., the coolingchannel can be machined into the outermost ring.

FIG. 5 shows another nickel-containing flange where additional ringshave been included in the flange to provide finer control over thecurrent distribution within the ring. In particular, in addition toinnermost ring 140 which comprises at least 80 wt. % platinum, thisembodiment includes high-temperature metal rings 141 and 142 which alsocomprise at least 80 wt. % platinum. In certain embodiments, theseadditional rings have the same composition as ring 140, although theycan have different compositions if desired.

The FIG. 5 embodiment also includes outermost ring 150 which comprisesat least 99.0 wt. % nickel as well as additional ring 151 whichsimilarly comprises at least 99.0 wt. % nickel. In certain embodiments,this additional ring has the same composition as ring 150, although itcan have a different composition if desired.

Although in FIG. 5, two additional platinum-containing rings and oneadditional nickel-containing ring are shown, it should be understoodthat more or less additional rings can be used in the practice of theinvention. Indeed, as illustrated in FIG. 4, the invention can bepracticed with just an innermost platinum-containing ring and anoutermost nickel-containing ring.

In FIG. 5, rings 140, 141, 142, 150, and 151 have different thicknesses.These thicknesses are chosen to control the current density as afunction of radial position. A number of considerations come into playin selecting these thicknesses. First, as discussed above, the primarygoal of direct resistance heating is to heat the molten glass in vessel10, not to heat the flanges supplying current to the vessel's wall.Accordingly, the current density in the flange should be less than thecurrent density in the wall. Second, the current density needs to becontrolled so that parts of the flange do not become overheated andthereby damaged. This is particularly a problem for those portions ofthe flange which experience higher ambient temperatures during use.

As a starting point for selecting ring thicknesses, it can be noted thata circular flange constructed of a single material having a constantthickness will have a current density that increases linearly withdecreasing radius, i.e., the current density will be the smallest at theouter edge of the flange and the greatest at the inner edge. To offsetthis effect, the thickness of the flange will typically increase as theradius becomes smaller. In terms of temperature, the ambient temperaturenormally drops as one moves outward from vessel 10 and thus currentdensity can be higher towards the outside of the flange where thechances of damage due to overheating are less. This also leads to aflange whose thickness becomes smaller as the radius increases. Suchreduced thickness is also desirable in terms of minimizing the amount ofmaterial used to construct the flange, especially in the case ofexpensive platinum-containing materials.

A further factor involves the resistivity of the material making up theflange, especially where more than one type of material is being used.The higher the resistivity, the greater the direct heating effect forthe same current density. Also, to obtain a bus bar effect, it can bedesirable for the outermost ring of the flange to have a substantialthickness so that the ring has a low resistance to circumferentialcurrent flow. More particularly, in certain embodiments, the variationin calculated radial current density i.e., the modeled current densityvariation; see below) around the circumference of the outermost ring isless than 50%.

In addition to these electrical considerations, the effects of operatingtemperature on the nickel-containing components of the flange also needto be considered. In general terms, suitable temperatures for thenickel-containing components of the flange are: (1) less than about 600°C. in normal operation with water cooling, (2) less than about 800° C.with air cooling, and (3) less than about 1000° C. uncooled. At about600° C. and below, nickel has a sufficiently low oxidation rate so thatflange lifetimes of three years or more can be achieved. At about 1000°C., the usable lifetime is less than 30 days. The lifetime at about 800°C. is between these values, and may be acceptable for some applications,especially if exposing the nickel to these temperatures allows aircooling to be used which can often be less complex than water cooling.

More generally, temperatures decrease in the refractory insulation asthe radial position from the axis of the glass-containing vessel isincreased. Temperatures likewise decrease with increasing radius of theflange. At some radial position on the flange, the temperature duringuncooled operating conditions drops below about 1000° C. Beyond thisradial position, nickel can safely be used for the flange material. Ifthe nickel temperature limits, e.g., about 600° C. for long life, about800° C. for intermediate life, or about 1000° C. for short periods oftime, are exceeded under any condition, the joint between nickel and thehigh-temperature metal used in the inner part of the flange must bemoved to a larger radius. Outward movement of the joint, of course,needs to be balanced against increased material costs since thehigh-temperature, and thus, high cost metal must then extend to a largerradius.

In practice, computer modeling will typically be used to take intoaccount the various factors involved in selecting the radii andthicknesses of the rings making up the flange. Such modeling can beperformed using commercially available or customized software packageswhich calculate electrical current flows for specified conductorproperties and geometries, as well as packages that model heat flows andcalculate temperature distributions for specified material propertiesand heat source/sink locations. For example, using such analyses, asuitable relationship for the thicknesses t's) of the rings of FIG. 5was found to be: t₁₄₀>t₁₄₁>t₁₄₂; t₁₅₀>>t₁₅₁; and t₁₄₀≈t₁₅₁, where rings140, 141, and 142 were made of 90 wt. % platinum and 10 wt. % rhodium,and rings 150 and 151, as well as main bus bar 170 and cooling tube 160,were made of nickel 200. Other relationships can, of course, be used inthe practice of the invention, the specific relationship which issuitable for any particular application of the invention being readilydetermined by persons skilled in the art from the present disclosure.

The rings and the bus bar used to construct the flange will typically befabricated from flat metal sheets, e.g., nickel 200 or nickel 201 sheetsfor main bus bar 170 and rings 150 and 151, and a platinum-rhodium alloye.g., 90 wt. % platinum and 10 wt. % rhodium) for rings 140, 141, and142. Metal sheets are commercially available in a limited set ofthicknesses. As a result, the thickness of the flange will change instepped amounts as one passes from one ring to the next. The steps arechosen so that the current density in each ring meets the specifiedcriteria see above) for the range of radial positions covered by thering. In particular, the joints between the rings occur at radialpositions such that the current density limits are not exceeded.

The joints between the rings are welded. The welds can be filleted toavoid re-entrant corners which can produce a locally high currentdensity that can cause a joint to overheat and fail. The innermost ringis joined to outer wall 12 of vessel 10, usually by welding. Again,filleting can be used to avoid re-entrant corners. The thickness of theinnermost ring is typically greater than the thickness of the vessel'swall 12, although other thicknesses can be used for the innermost ringif desired, e.g., the thickness of the innermost ring can be equal to orsmaller than the thickness of wall 12.

FIG. 6 shows an additional embodiment of the invention which includes anintermediate ring 190 which comprises a platinum-nickel alloy. Asdiscussed above, to achieve a long service life, the rings of the flangewhich comprise 99.0 wt. % nickel need to be located at positions wherethe operating temperature is below about 600° C. Above this temperature,nickel has a high rate of oxidation which limits its useful life.

As reported in the scientific literature, nickel-platinum alloys formadherent and protective scales when exposed to high temperatures in airand therefore oxidize at much lower rates than pure nickel. See, forexample, C. Wagner and K. Grunewald, Z. Physik. Chem. B, 1938 vol 40 p455; O. Kubaschewski and O. von Goldbeck, J. Inst. Metals 1949 vol 76 p255; D. E. Thomas, “Discussion—On the Mechanism of Oxidation ofNickel-Platinum Alloys”, J. Inst. Metals, 1949), vol 76 pp. 738-741; andO. Kubaschewski and B. E. Hopkins, “Oxidation of Metals and Alloys”,Academic Press, 1962.

In particular, the oxidation rate of platinum-nickel alloys is reportedto start to decrease when the platinum content exceeds 50 mol %, i.e.,77 wt. % for an alloy which contains only platinum and nickel. See C.Wagner, “Theoretical Analysis of the Diffusion Processes Determining theOxidation Rate of Alloys”, J. Electrochem Soc., vol. 99 (1952), pp369-380.

As a result of their lower rates of oxidation, nickel-platinum alloyscan be used at higher temperatures than pure nickel. This, in turn,means that the amount of expensive, high-temperature metal used in aflange can be reduced since rings comprising a nickel-platinum alloy canbe used at locations where a high-temperature metal would otherwise beneeded.

Table 1 sets forth room temperature electrical resistance and thermalconductivity data for platinum, nickel, and representativenickel-platinum alloys. These data are from the ASM Handbook, Volume 2,“Properties and Selection: Nonferrous Alloys and Special-PurposeMaterials”, 1990, p 713 and Y. Terada, K Ohkubo, and T. Mohri, “ThermalConductivities of Platinum Alloys at High Temperatures”, Platinum MetalsReview, 2005, vol 49, pp 21-26.

As these data show, nickel-platinum alloys have lower thermalconductivities and higher resistivities than platinum or nickel. Thesedifferences provide additional degrees of freedom which can be used tooptimize power input and minimize heat loss. Moreover, a wide range ofplatinum-nickel alloys can easily be made since platinum and nickel formsolid solutions over the entire composition range. It should be notedthat the lower thermal conductivities of the alloys may be particularlyuseful since meaningful amounts of heat loss from vessel 10 can occurthrough water-cooled flanges composed of materials having high thermalconductivities. Excessive heat loss by this mechanism can createproblems with glass quality. By using rings composed of platinum-nickelalloys, such heat losses can be reduced.

In terms of cost, it should be noted that an alloy comprising 90 wt. %platinum and 10 wt. % nickel is commercially available. Moreover, aswith nickel, alloys of platinum and nickel are readily weldable toplatinum and platinum-rhodium, which facilitates assembly of the flange.

From the foregoing, it can be seen that embodiments of the invention caninclude some or all of the following features:

a) A disc-shaped metal flange that, during use, is welded to ahigh-temperature metal vessel high-temperature metal tube) whichcontains and/or carries molten glass.

b) A high-temperature metal comprising at least 80 wt. % platinum thatforms the inner radial portions of the flange.

c) At least 99.0 wt. % nickel that forms the outer radial positions ofthe flange.

d) The thickness of the flange, specifically, the high-temperature metalpart of the flange, can be reduced as the radius increases to minimizematerial usage while keeping electrical current density below a chosenlevel.

e) The outer portion of the flange can include a circumferential bus barwhich comprises at least 99.0 wt. % nickel; the bus bar dimensions arechosen to evenly distribute current around the circumference of theflange; the bus bar can be configured as the outermost ring of theflange.

f) A cooling channel that comprises at least 99.0 wt. % nickel can bewelded to the circumferential bus bar or, alternatively, machined intothe bus bar.

g) A linear main) bus bar can be attached to the circumferential bus barat one radial position to carry current from an electrical power circuitto the flange.

h) When used with water cooling, the transition between the innerhigh-temperature metal rings) and the outer 99.0 wt. % nickel rings) canoccur at a temperature less than or equal to 600° C.; when there is aloss of coolant, the temperature at the transition can remain below1000° C.

i) The flange can include one or more intermediate rings which comprisea platinum-nickel alloy, e.g., an alloy containing at least 77 wt. %platinum.

A variety of modifications which do not depart from the scope and spiritof the invention will be evident to persons of ordinary skill in the artfrom the foregoing disclosure. The following claims are intended tocover the specific embodiments set forth herein as well as suchmodifications, variations, and equivalents.

TABLE 1 Electrical Resistivity Thermal Conductivity Nano-ohm meterW/meter * K Platinum 98 71.1 Pt—5% Ni 236 Estimate - 29 Pt—10% Ni 298Estimate - 23 Pt—15% Ni 330 Estimate - 21 Pt—20% Ni 350 Estimate - 20Nickel 68 88  

What is claimed is:
 1. A flange for use in direct resistance heating ofa vessel which, during use, carries molten glass and which comprises anexterior wall which comprises at least 80 wt. % platinum, said flangecomprising: (a) a plurality of electrically-conductive rings which,during use, form a conductive path for carrying current to the exteriorwall, said plurality of rings comprising: (i) an innermost ring which isjoined to and electrically connected with the vessel's exterior wallduring use of the flange; and (ii) an outermost ring which receiveselectrical current during use of the flange; and (b) a cooling channelassociated with the outermost ring through which a cooling fluid flowsduring use of the flange; wherein: (i) the innermost ring comprises ahigh-temperature metal which comprises at least 80 wt. % platinum; (ii)the outermost ring comprises at least 99.0 wt. % nickel; and (iii) theplurality of electrically conductive rings comprises a ring which (A) islocated between the outermost and innermost rings and (B) comprises aplatinum-nickel alloy which: (I) comprises more than 50 mole percentplatinum, (II) comprises 10-20 wt. % nickel, and (III) during use of theflange, forms an adherent and protective scale which reduces theoxidation rate of the platinum-nickel alloy compared to pure nickel. 2.The flange of claim 1 wherein the platinum-nickel alloy comprises atleast 77 wt. % platinum.